Hi Michael! Thanks for posting. We don't have an automatic feature/shape that creates a border surrounding your diagram.

We’re very interested in your feedback and committed to continually improving our products. If you’re willing to share, we’d love to hear more details about your use case or what you’d like to see in this experience posted in the Product Feedback section of the community - from here, other users can upvote it and add details of their own. For more information on how Lucid manages feedback via this community, take a look at this post.

In the meantime as a workaround the best way to create a quick line border would be to grab one of our rectangular shapes (i.e. process shape) or one of our container shapes make it transparent and drag it out to be as large as you'd like the border to be.

Additionally you can look through our available shape libraries by selecting "m" while in a document or by selecting "shapes" on your left toolbar. 

Hope this helps!